Conventionally, in this type of electromagnetic flow meter, an AC excitation current lex is supplied to an excitation coil that is disposed so that the direction wherein the magnetic field thereof is produced is perpendicular to the direction of flow of a fluid flowing within a measuring tube, and an electric signal that includes a signal EMF that is produced between a pair of electrodes that are disposed within the measuring tube, perpendicular to the magnetic field that is produced by the excitation coil, is detected as an AC current signal, and this detected AC current signal is sampled and subjected to signal processing to produce a measured flow rate.
FIG. 3 is a illustrates schematically a conventional electromagnetic flow meter. In this figure: 10 is a detecting device for applying a magnetic field to a fluid that flows within a measuring tube 10C based on the AC magnetic excitation electric current lex, and detecting and outputting, as an AC flow rate signal, an electric signal that includes a signal EMF that is produced in the fluid; and 11 is a converting device for not only outputting the magnetic excitation electric current lex to the detecting device 10, but also performing signal processing on the AC flow rate signal from the detecting device 10 to calculate and output the flow rate of the fluid flowing within the measuring tube 10C.
In this electromagnetic flow meter, the magnetic excitation portion 8 of the converting device 11 outputs a square wave AC magnetic excitation electric current lex of a specific frequency based on a magnetic excitation signal 9B from a switching portion 9. A magnetic excitation coil 10D of the detecting device 10 is magnetically excited by the AC magnetic excitation electric current ilex from the converting device 11, to apply a specific magnetic field to the fluid that flows within the measuring tube 10C, to thereby produce a signal EMF having an amplitude that is dependent on the flow speed of the fluid. Additionally, an electric signal including the signal EMF is detected by the electrodes 10A and 10B, and outputted to the converting device 11 as an AC flow rate signal.
In the converting device 11, in an AC amplifying portion 1, the AC flow rate signal from the detecting device 10 is amplified, and outputted as an amplified AC flow rate signal 1A. In a sample hold portion 3, the positive side and the negative side of the AC flow rate signal 1A from the AC amplifying portion 1 are both sampled, through switches 31S and 32S of sampling circuits 31 and 32 being controlled based on a sampling signal 9A from the switching portion 9, to be outputted from an operational amplifier 33 as a DC flow rate signal 3A.
The DC flow rate signal 3A that is outputted from the sample hold portion 3 is converted into a digital signal by an A/D converting portion 5. A calculating/processing portion 6 accepts the digital signal from the A/D converting portion 5 and calculates the desired measured flow rate value through performing specific calculation processes, and, in an outputting portion 7, converts into a specific signal, which is outputted.
FIG. 4 is a timing chart illustrating the sampling operation for the electromagnetic flow meter illustrated in FIG. 3, wherein: FIG. 4 (a) is the magnetic excitation signal 9B from the switching portion 9; FIG. 4 (b) is the AC flow rate signal 1A outputted to the sample hold portion 3; FIG. 4 (c) is the switching operation of the switch 31S that operates based on the sampling signal 9A from the switching portion 9; FIG. 4 (d) is the switching operation of the switch 32S that operates based on the sampling signal 9A from the switching portion 9; and FIG. 4 (e) is the DC flow rate signal 3A that is outputted from the sample hold portion 3.
In this case, the sampling intervals by the switches 31S and 32S (the intervals indicated by the diagonal lines in FIG. 4 (b)) are established in the vicinity of the ends of each pulse of the magnetic excitation signals 9B (the AC flow rate signals 1A) in consideration of the stability of the waveform, and in the sample hold portion 3, the switches 31S and 32S are each ON during only the sampling intervals, to integrate and output the AC flow rate signals 1A as the DC flow rate signal 3A.
In this electromagnetic flow meter, there are many types of noises, such as spike noise and low-frequency noise, as noises that are mixed in from the electrodes 10A and 10B. In particular, the impact of the spike noise is remarkable, and ignoring the spike noise may lead to a decrease in measurement accuracy.
In order to eliminate the impact of the spike noise in, for example, the electromagnetic flow meter disclosed in Japanese Unexamined Patent Application Publication 2000-292230, a noise eliminating circuit for eliminating the spike noise is provided between the AC amplifying device 1 and the sample hold portion 3.
However, in a structure wherein the noise eliminating circuit is disposed between the AC amplifying device 1 and the sample hold portion 3, if noise incurs between the sample hold portion 3 and the AD converting portion 5, then it will not be possible to eliminate the noise that has incurred.
The present invention was created in order to solve the problem as set forth above, and the object thereof is to provide an electromagnetic flow meter able to eliminate noise that incurs between the sample hold portion and the A/D converting portion.